In certain offshore applications, keel guides are mounted to various vessels or platforms to guide risers extending to subsea locations. The keel guides restrain the upper end of the risers against lateral motion, thus preventing the risers from interfering with each other or with the vessel or platform. Generally, a keel guide comprises a cylindrical member or “can” which is attached to the hull of the vessel or platform with an appropriate bracket.
Risers are permitted to move vertically within the keel guide to compensate for motion of the vessel or platform. Each riser is equipped with a keel joint designed to ride within the keel guide. Generally, the keel joint comprises a pipe section of increased thickness to withstand the bending loads exerted on the joint by the keel guide. The keel joint may be provided with an outer wear sleeve along the portion of the joint which contacts the keel guide.
In many applications, a tieback connector is coupled to a lower end of the riser and moved to the seabed as the riser is lowered. However, such connectors may tend to be too large to pass through the keel guide of nominal size. Accordingly, the riser is run outside of or offset from the keel guide and moved into the keel guide in a later procedure. In some applications, for example, the keel guide is formed with a slot, and once the connector has passed the keel guide, the vessel or platform is translated toward the riser until the riser passes through the slot and into the keel guide. The riser is then moved vertically until the keel joint enters the keel guide. The outer diameter of the keel joint is larger than the width of the slot to restrain the keel joint within the keel guide.
In some applications, the riser is lowered until the tieback connector is below the keel guide. At this point, the vessel or platform is translated, until the riser moves through the slot in the keel guide. The riser is then lowered and positioned until the keel joint is within the keel guide, the riser is tensioned and the keel joint remains positioned in the keel guide.
Translation of the vessel or platform to the riser coupled with subsequent movement of the keel joint into the keel guide is a costly and time-consuming process. Additionally, such an approach typically requires the cutting of a slot into the platform structure of sufficient width to permit the passing of the riser from a position external to the keel guide to a position within the keel guide.